For years, severe mortality of brown trout (Salmo trutta) in late summer has been reported in the prealpine river systems of Austria. For an initial understanding of the potential reasons for this mortality, brown trout were exposed to water under differing conditions from an affected river system. Blood parameters, histology of various organs, hepatic ethoxyresorufin-O-deethylase (EROD) activity, and occurrence of ectoparasites and fish diseases were investigated. In brown trout exposed to water from the affected river, concentrations of peripheral blood lymphocytes, granulocytes, plasma immunoglobulin and plasma lyszoyme activity decreased significantly, indicating a reduction in unspecific, innate and specific, adaptive immune response. Erythropoietic and thrombopoietic functions were also disturbed, with fish mortalities up to 100%. From the results of the present study it seems unlikely that ectoparasites, diseases, or effluent water from sewage plants were causative for the observed phenomenon. However, brown trout maintained in river water and in the dark survived longer than those under natural light conditions, and their immune systems were less drastically affected. As periods of high solar UV-radiation occurred during the experiment and the Traun River water temperature was quite variable, these factors might have had an effect on immune systems. Laboratory studies were therefore conducted to ascertain whether water temperature variations and/or UV could indeed affect the immune system. The tested temperature variations (10°C/d) suppressed the brown trout immune system, whereas UV radiation (0.045 mW/m2 for 6.5 h per day) had no effect. However, UV-radiation did enhance the immune-suppression effect of variable temperatures.